1. Field of the Invention
This invention involves communications methods that a wireless remote station uses to gain access to a base station in a discrete multitone spread spectrum communications system.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems to efficiently make use of limited bandwidths. The protocol uses a unique code to distinguish each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel.
The personal wireless access network (PWAN) system described in the referenced Alamouti, et al. patent application, incorporated herein by reference, uses a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) to provide efficient communications between a base station and a plurality of remote units. In this protocol, the user's data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spreading codes that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers with the real component acting to modulate the amplitude of a tone while the complex component of the weight acts to modulate the phase of the same tone. Each tone in the weighted tone set bears the same data signal. Plural users at the transmitting station can use the same tone set to transmit their data, but each of the users sharing the tone set has a different set of spreading codes. The weighted tone set for a particular user is transmitted to the receiving station where it is processed with despreading codes related to the user's spreading codes, to recover the user's data signal. For each of the spatially separated antennas at the receiver, the received multitone signals are transformed from time domain signals to frequency domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna element. The values of the despreading weights are combined with the received signals to obtain an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) equally spaced in 8 MHz of available bandwidth in the range of 1850 to 1990 MHz. The spacing between the tones is 3.125 kHz. The total set of tones are numbered consecutively from 0 to 2559 starting from the lowest frequency tone. The tones are used to carry traffic messages and overhead messages between the base station and the plurality of remote units. The traffic tones are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones.
In addition, the PWAN system uses overhead tones to establish synchronization and to pass control information between the base station and the remote units. A Common Link Channel (CLC) is used by the base to transmit control information to the Remote Units. A Common Access Channel (CAC) is used to transmit messages from the Remote Unit to the Base. There is one grouping of tones assigned to each channel. These overhead channels are used in common by all of the remote units when they are exchanging control messages with the base station.
In the PWAN system, Time Division Duplexing (TDD) is used by the base station and the remote unit to transmit data and control information in both directions over the same multi-tone frequency channel. Transmission from the base station to the remote unit is called forward transmission and transmission from the remote unit to the base station is called reverse transmission. The time between recurrent transmissions from either the remote unit or the base station is the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted in each burst using multiple tones. The base station and each remote unit must synchronize and conform to the TDD timing structure and both the base station and the remote unit must synchronize to a framing structure. All remote units and base stations must be synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time. When a remote unit initially powers up, it acquires synchronization from the base station so that it can exchange control and traffic messages within the prescribed TDD time format. The remote unit must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote is operating at the same frequency and phase as the base station.
When a caller at a remote unit goes off-hook, an access request message is sent by the remote unit over the Common Link Channel (CLC) to the base station during the reverse TDD interval when all of the remotes are allowed to transmit. If more than one remote unit sends a message over the CAC channel during the same reverse TDD interval, there is a collision of the signal tones. If the base station receives the combined signal from the collided tones, the signal will not be intelligible. In that case the base station will reply with a negative acknowledgement signal. Alternately, if the base station never receives the collided signals, the absence of an acknowledgement signal from the base station will be inferred by both remote units as a collision. In either case, the remote units in the present PWAN system will delay repeating their transmissions by a random interval. This collision detection multiple access technique is generally known as the aloha protocol. Each remote unit will delay retransmission by a random interval, known as a back-off interval, that is usually different for the two units. The remote unit whose random interval is the first to expire, will be the first to retransmit its message.
A problem arises when the collision is not between the transmissions from two remote units, but instead is between a transmission from one remote unit and a noise burst. Noise bursts are typically of a longer duration than the typical back-off interval of the standard aloha protocol. If the remote unit infers from the base station's negative acknowledgement signal or from the lack of an acknowledgement signal that there has been a collision with a transmission from another remote station, the remote unit will not delay long enough to avoid a second collision with the noise burst when it retransmits its signal. However, the possible solution of merely lengthening the aloha back-off intervals for all detected collisions would unnecessarily delay most retransmissions after normal collisions with other remotes.